Cytochrome P450s are a group of enzymes capable of metabolizing foreign chemicals of diverse structure. A single form of P450 can oxidize a large number of different compounds and thus a finite number of P450s can deal with numerous chemicals ingested in the diet, in the form of drugs and environmental pollutants, and inhaled from the air. P450s require the enzyme NADPH-P450 oxidoreductase and, in some cases, catalytic activities are potentiated by cytochrome b5. The structure of mammalian P450s is unknown since none of these membrane-bound enzymes have been crystallized. This contrasts the soluble bacterial P450cam whose substrate-free and substrate-bound three dimensional structures are known. The amino acid residues responsible for catalytic activities and substrate specificities of the mammalian P450s are being elucidated by CDNA expression in conjunction with site directed mutagenesis studies. It is difficult, however, to select potential sites to change amino acid residues by sequence comparisons or by extrapolation of a theoretical mammalian P450 structure based on the bacterial enzyme. We have discovered allelic variants of rat P450s that contain different catalytic activities and only one or a few amino acid differences. These have been exploited to determine those residues responsible for catalytic activities of mammalian P450s towards drugs, chemical carcinogens, and other chemicals.